1. Field of the Invention
This invention relates generally to a stabilizer apparatus for logging-while-drilling ("LWD") or measurement-while-drilling ("MWD") applications, and particularly to a stabilizer including a plurality of outwardly extending blades on a tubular body that can be mounted to a standard drill collar and rigidly secured at a selected longitudinal and rotational position thereon.
2. Description of the Related Art
Stabilizers are used in a drill string primarily for the purpose of providing a predetermined radial spacing of the longitudinal axis of the drill string with respect to the borehole wall. A stabilizer can be either full-gauge so that the outer diameter of its blades is substantially the same as the gauge diameter of the bit, or somewhat under-gauge so that the outer diameter of its blades is less than the gauge diameter of the bit. The use of various combinations of full and/or under-gauge stabilizers, and the longitudinal spacing thereof along the drill string above the bit, is one of various methods which can be used to control the direction the borehole takes during drilling.
In many instances it is desirable to mount a stabilizer on a "slick" drill collar, that is a drill collar having no special machining on its outside where the stabilizer is to be located. Such machining greatly increases the cost of the collar, reduces its versatility, and can substantially increase fishing and wash-over costs to the operator under circumstances mentioned below. Moreover, in highly deviated or horizontal wells a non-slick collar provides more drag and less weight-on-bit, which are other important disadvantages. In a principle aspect, the present invention is directed toward the provision of a stabilizer that can be slidably mounted on and rigidly attached to a slick drill collar in order to obviate the various disadvantages noted above.
Where a stabilizer is used in association with a logging-while-drilling (LWD) or a measurement-while-drilling (MWD) tool, it often is desirable to locate one or more transducers that operate to measure a characteristic property of the earth formations which have been penetrated by the bit within or underneath the blades of the stabilizer. The primary purpose of such location is to provide a physical contact between the transducer and the earth formation. This causes the drilling mud to be temporarily displaced by the stabilizer blades to provide a measurement environment that is substantially free of borehole effects which otherwise can have an adverse impact on the quality of the measurement. Such borehole effects include the standoff between the tool and the earth formations, and drilling mud and mudcake located between the tool and the earth formations. In some applications, it is also desirable to locate the transducers as close to the bit as possible so that measurements can be made before mud filtrate has invaded the formation and before a substantial mudcake has built up on the borehole walls.
Examples of measurements that can be made while drilling are the density and porosity of the formation such as disclosed in U.S. Pat. Nos. 4,814,609 and 4,879,463, which are assigned to the assignee hereof and hereby incorporated herein by reference. For example in the measurement of formation density, a plurality of apertures filled with gamma-ray transparent plugs or windows are preferably provided in the outer body of the tool in registration with the source and detectors. In order to minimize the borehole effects described earlier herein, the source and detectors are preferably housed underneath a stabilizer having similar apertures filled with gamma-ray transparent plugs that are in registration with the tubular body apertures and the source/detectors behind them. As will be appreciated by those skilled in the art, controlling and maintaining the shape, size, spacing, and alignment of these apertures with respect to the underlying tool components (e.g. nuclear sources, detectors, electronics, etc.) can be critical in obtaining accurate and reliable measurements. Where recovery of stuck drill collars is attempted by fishing and washover operations noted above, the costs thereof may be drastically reduced where a slick drill collar is being used. Additionally, the full range of fishing, washover and milling operations is made possible by prior removal of any nuclear logging sources present in the MWD or LWD tools in the bottom hole assembly by methods disclosed in commonly assigned U.S. Pat. Nos. 4,814,609 and 5,184,692 which are incorporated herein by references.
Another downhole formation measurement that can be made with an LWD tool is electrical resistivity or its reciprocal, conductivity. In commonly-assigned U.S. Pat. application No. 07/786,137 filed Oct. 31, 1991 and incorporated herein by reference, a preferred embodiment of the LWD tool measures formation resistivity by employing a current emitting electrode and one or more current receiving electrodes mounted on a stabilizer. Since the current is emitted and received through spaced openings in the blade, again the axial and rotational alignment of the blade with respect to the electronics and wiring contained in the underlying LWD tool is of extreme importance. Unless such alignment is maintained in the borehole during drilling, the measurements can be severely attenuated, if not disrupted altogether.
A stabilizer assembly that with some modification has been used in LWD operations is disclosed in U.S. Pat. No. 4,275,935 issued Jun. 30, 1981 to Charles M. Thompson et al. and entitled "Drilling Stabilizer". The stabilizer includes a sleeve having blades and is longitudinally split along one of its blades so that it can be slipped onto a drill collar. Then a plurality of bolts are placed in holes through the split blade, and are tightened to reduce the inner diameter of the stabilizer and thus obtain a friction fit around the collar. However, since this stabilizer is not circumferentially continuous, the bolts, which are highly stressed, tend to fatigue after a limited number of cycles. Additionally, the sensitivity of the friction fit to the condition of the drill collar can reduce the holding capability of this stabilizer and result in movement of the stabilizer with respect to the collar.
U.S. Pat. No. 3,164,216 discloses a drill pipe protector having hinged halves that encircle the drill string and are pinned in place. Radial screws which engage metal tabs that pinch inward against an internal rubber sleeve are used to hold the protectors in place. However, it is believed that in a real drilling environment a rubber sleeve may not be sturdy enough to adequately prevent movement of a stabilizer for MWD and LWD applications, particularly under conditions of higher down hole temperatures, no matter how much it is pinched. U.S. Pat. No. 2,813,697 also discloses a stabilizer having screws which go through overlapping tongues in a metal sleeve which holds an elastomer stabilizer around a reduced diameter section of a drill collar. These screws are merely for assembly and disassembly purposes and the stabilizer is held by shoulders formed on the collar.
Other patents such as U.S. Pat. Nos. 4,101,179 and 3,916,998 show stabilizers that have conical surfaces which cause one or more movable members to frictionally engage the outer surface of a drill collar. However, mere frictional engagement of a small ring, particularly over a small area, may be insufficient to hold the stabilizer in position for MWD and LWD applications. U.S. Pat. Nos. 4,011,918, 3,945,446 and 3,938,853 relate to "shrink-fit" stabilizers which require machining of accurate conical surfaces, as well as complicated and expensive hydraulic equipment that is used to attain the shrink fit. Each of these devices has drawbacks which are eliminated by the present invention.
A general object of the present invention is to provide a new and improved stabilizer that can be positioned on a drill collar and attached thereto in a manner which obviates the disadvantages noted above.
Another object of the present invention is to provide a new and improved stabilizer that can be positioned on a slick drill collar and has radially energized dies that grip outer surfaces of the drill collar to positively attach the stabilizer thereto.
Still another object of the present invention is to provide a new and improved stabilizer of the type described which can be secured on a drill collar or other tubular housing at a precise longitudinal and rotational position in order to cooperate with various sources and detectors that may be found on an LWD or MWD tool.